When they did this on Junkyard wars, they removed the head off a 4 cyl engine and replaced it with a flat sheet of steel. Holes were drilled for pipes and check valves were installed.

I think the idea of the VW engine is best, but the amount of power to run this would be pretty high, something in the range of 5+ HP 220 volt motor.

 

I once saw a flathead V8 converted into an air compressor at a WWII vintage shipyard/drydock in Seattle (Lake Union Drydock on Eastlake). I think it was used to pump air into the drydock to raise it up. It was still in service in 1979.

One side of the engine ran as a four cylinder engine. The other side was rigged as an air compressor.

 

How much pressure are you planning to get out of this compressor? I could see how these things could work at low pressures, but if you're using an engine with, say, 8 to 1 compression, 8 X 14.7 psi == 117 psi being the absolute limit that this compressor could produce. It would be increasing awfully slowly once it got close to this limit too. If you could somehow rig up a diesel engine though...:-)

Or am I missing something here?

 

Ooh, I just thought of something. You could make this into sprt of a "two-stage" setup. If you could get the valves in the head to work where they let you compress air through the spark plug hole, like some of the others suggested, then I think this could work. Suppose you were using a four-cylinder engine. You would set up three of the cylinders as suggested, with piping out the spark plug holes and check valves to prevent leakage back into the cylinders. These three pipes would route their air to a relatively small intermediate tank, like a gallon sized or something. Coming out of this small tank would be yet another pipe. This pipe would T right next to your fourth cylinder. Just before the T would be installed a check valve oriented the opposite direction as your other three, i.e. to prevent air from being compressed back into your small tank. It would T off one way into the fourth cylinder and the other way to your large reservoir tank, with a check valve installed as close to your cylinder as possible. All valves on this cylinder would be closed at all times. This would really get your max pressure up there. Conservatively speaking, using the 8 to 1 compression engine, if you could maintain 4 times atmospheric pressure in your intermediate reservoir (I don't know what the actual number is, this seems reasonable) then your new theoretical max pressure would be 4 X 8 X 14.7 psi == 470 psi. That's a lot better than it was

I think this would work. Did I explain it so that it makes any sense? Now this has got me intrigued...

 

Dont knot too much about this subject, but i once saw a guy who had a older car with a v8. he had a fitting that screwed in the spark plug hole on 1 cyl. that had an air line ran off of it. When he needed to pump his tire up or use an air tool he would pull the plug and hook his line up. Dunno how much pressure it would make but it was enough to inflate a tire and power an impact wrench. Not sure how safe this is though. I would prob. just buy a cheap compressor.

 

"your new theoretical max pressure would be 4 X 8 X 14.7 psi == 470 psi. That's a lot better than it was "

I don't get your theory on this? I didn't know compression ratio had anything to do with maximum developed pressure. I thought it was a strength issue. As in how strong the engine/motor was turning the compressor, and how much the compressor unit itself could actually take before blowing up.

Compression ratio is the ratio of cylinder volume when the piston is down in the cylinder, vs when it is up with both valves shut(compression stroke). But in a compressor, there would never be a time when both valves are shut. During the up stroke, one valve would be open to the air tank it was supplying.

 

All I am saying is that if you use one-way check valves to keep the air from flowing out of the big tank, you will eventually reach a point where the compression produced in the cylinder won't be enough to open the check valve against the pressure in the tank. If you had a two-way valve of some sort (and I don't know what actual compressors use) that could expose your cylinder to the compressed air in the tank, then the sky's the limit. What I was trying to do with my "two-stage" idea is to fill the cylinder with pressurized air to begin with. Then there would be a lot more cylinder pressure to force the air past the one-way check valve. If you had a two-way valve this wouldn't be necessary.

Or so I think...

 

So the 3 cyls would act like a supercharger for the 4th one, which would compress the air even more.

 

I think that the half-engine deal would be the best way to do it, if you could manage to get it done for less than a cheap air compressor would cost (but that's no fun!). Otherwise I imagine that it would need a monster electric motor to spin it (the stock starter motor hooked up to an power substation might work for a few exciting seconds...).

 

Gas engines are four stroke but the air compressor would be two stroke so it would pump air at twice the rate that the engine used air. The spark plug hole idea might not work if the hole is too small.

I suppose that a flathead is ideal because you could fabricate a cylinder head that had a bigger exit hole.